Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Table V. Sodium Borohydride-Reduced Electroless Nickel Bath Nickel chloride Sodium hydroxide Ethylenediamine, 98% Sodium borohydride Thallium nitrate pH Temperature C/240O 31 g/L 42 g/L 52 g/L 1.2 g/L 0.022 g/L >13 93-95° C (200-205° F) structural changes of the base substrate. Additionally, low temperatures are used (116O F) to relieve any hydrogen embrittlement that may be produced from pretreatment cycles or subsequent electroless nickel deposition. Postbaking of the deposit produces marked structural changes in hardness and in wear and abrasion resistance. Depending upon the temperature, bath com- position, and phosphorus content, this postbake cycle will totally change the ini- tial amorphous structure, resulting in nickel phosphide precipitation creating a very hard matrix. Complete precipitation of nickel phosphides does not occur at temperatures significantly below 399° C (750° C (482° F). In general, deposits with 9.0% phosphorus and above tend to produce lower as-deposited hardness values, but give slightly higher hardness when post-heat-treated. The coating will discolor above 250° F) in an air atmosphere. Prevention of coating discoloration can be accomplished in a vacuum, inert, or reducing atmosphere oven. Physical properties affected by the post-heat-treatment include increasing hardness, magnetism, adhesion, tensile strength, and electrical conductivity while decreas- ing ductility, electrical resistivity, and corrosion resistance. Thickness of the nickel-phosphorus deposit generally ranges from 2.5 to 250 µm (0.1-10.0 mil). Deposits less than 2.5 µm and greater than 625 µm are currently and successfully being performed. The latter being typical of repair or salvage appli- cations. Thickness measurements can be carried out with electromagnetic devices (eddy current), micrometers, coulometrics, beta backscatter, and X-ray fluorescence. Table IV gives an example of an acid hypophosphite-reduced bath. Alkaline nickel-boron solutions utilize the powerful reducing agent, sodium borohydride, to produce a deposit containing 5-6% boron and 94-95% nickel by weight. These highly alkaline solutions operate at a pH of 12.0-14.0 and tem- peratures of 90-95° C (195-205° F). These baths tend to be less stable because of their high alkalinity, and bath decomposition may occur if the pH falls below 12.0. Complexing agents such as ethylenediamine are used to prevent precipitation of nickel hydroxide. As-deposited hardness values of 650 to 750 VHN are typical. Table VI. Dimethylamine Borane-Reduced Electroless Nickel Bath Nickel sulfate Sodium acetate n-Dimethylamine borane (DMAB) Lead acetate pH Temperature 364 25 g/L 15 g/L 4 g/L 0.002 g/L 5.9 26° C (78° F)

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